One of typical devices as described above is so called a proportional pressure control valve or a linear solenoid valve in which a solenoid device is employed as an electric driver. The linear solenoid valve delivers a pressure at an output port. The pressure is substantially proportional to a current level of the solenoid device. The linear solenoid valve is applied to deliver a predetermined fluid pressure which is proportional to a level of an electric signal to a fluid circuit element or a fluid responsive device, for example, a shock absorber of a vehicular suspension.
For example, Japanese Laid-Open Patent Application No. 106,133/1988 discloses a pressure control system in which a turning pattern of a vehicle is determined on the basis of a steering angle and a steering angular velocity, and is utilized to modify a constant of proportionality or gain which is to be applied to a corrected suspension pressure which is determined in proportion to a lateral acceleration prevailing on the vehicle, and a pressure control valve for supplying the corrected suspension pressure to a suspension. The pressure control valve has a high pressure port communicated with a high pressure piping, a low pressure port communicated with a return piping to a reservoir, an output port which supplies the corrected pressure to the suspension, a spool which is driven in a direction by a pressure applied from the output port to an end of the spool to decrease a flow rate from the high pressure port to the output port while to increase a flow rate from the output port to the low pressure port and a solenoid for driving the spool in an opposite direction through a coiled spring to increase the flow rate from the high pressure port to the output port while to decrease the flow rate from the output port to the low pressure port. The spool assumes a position where the pressure at the output port is balanced with a driving force which depends on a current level used to energize the solenoid. Thus a pressure corresponding to the current level appears at the output port and is applied to a shock absorber of a suspension.
Japanese Laid-Open Patent Application No. 122717/1989 discloses a pressure control valve having a target pressure space and a needle valve for determining a pressure in the space between the spool and a plunger of the solenoid. The current level which energizes the solenoid determines the pressure in the target pressure space which in turn applied to the spool.
The spool of the pressure control valve has an annular groove 91 communicating with an output port 84 at a center portion of the spool 90 as shown in FIG. 10a. The spool 90 assumes a position where a force F.sub.O applied to a left end of the spool 90 by a pressure transmitted from an output port 84 is balanced with a force F.sub.i applied to a right end of the spool 90 by the solenoid or the pressure in the target pressure space. Because, when F.sub.O exceeds F.sub.i, the spool 90 is driven to the right, allowing the groove 91 to communicate with a low pressure port 85, thus causing the pressure of the output port 84 to decrease, which in turn decreases F.sub.O. While when F.sub.O decreases under F.sub.i, the spool 90 is driven to the left, allowing the groove 91 to communicate with a high pressure port 82, thus causing the pressure of the output port 84 to increase, which in turn increases F.sub.O. With the movements of the spool 90 as described, the pressure of the output port 84 is determined to that is substantially proportional to F.sub.i which is controlled with the current level of the solenoid.
Various sensors detect a height of a car body, a longitudinal acceleration and a lateral acceleration prevailing on the vehicle, a steering angular velocity and so on. An electronic controller detects or presumes a change of an attitude of the car body for calculating a pressure correction value. The controller adjusts the current level of the solenoid for correcting or compensating the change of the attitude of the car body. In this manner, when a dive or roll of the car body is presumed, the controller increases the pressure of one or more suspensions which may shrink by the dive or roll while decreases the pressure of remaining one or more suspensions which may extend by the dive or roll, for preventing an actual occurrence of the dive or roll of the car body. The other electronic controller determining a target height of the car body responding to a vehicle speed detected or an input operation of a vehicle driver, calculates a pressure correction value. The electronic controller adjusts the current level of the solenoid for compensating the pressure correction value.
Assuming that the current level of the solenoid is changed as a sine curve, the pressure at the output port 84 changes as shown in FIG. 10b. The pressure curve shown in FIG. 10b includes sharp fluctuations (higher harmonics). Also acute fluctuations of the pressure at the output port 84 occurs when the current level is maintained constant and the spool 90 moves to the right or left due to an acute rise or fall of the pressure at the output port 84 when a wheel of the vehicle suddenly rises or falls. The sharp or acute fluctuations of the pressure at the output port 84 may apply pressure impulses to a fluid circuit and the shock absorber of a suspension which are connected to the output port 84 of the pressure control valve. The pressure impulses may cause noises or vibrations and may degrade an endurance of the fluid circuit.